I too give the WNV vaccine myself.. I had for years.. I always choose to give it single, I choose most of my vaccines this way and I space them out, so I know that if they have a reaction then I know what the cause was..
I had a filly, at the time 2 years old.. WNV.. an hour later.. down on the ground, swollen muzzle and couldn't breathe.. Fortunately I had epinephrine.. it saved her life.. it took three treatments until the vet came (30 minutes).. Then a lot of stall rest, TLC, soaked feeds and a few days to get her over this reaction....
My advice.. if you vaccinate yourself.. PURCHASE epinephrine.. and know the approx. weight of your horse and watch them.. a close eye for hours and hours after being vaccinated..
Of course keep an eye on the injection site and them personally for several days, but in the beginning.. it can takes hours for a reaction.. not minutes..